Where did my texture go after selecting Use Nodes?
Materials created for Blender Internal are not compatible with Cycles. Blender Internal uses an entirely different nodes system, stemming from the fact that it is an entirely different rendering engine and rendering images via an entirely different method.

To use your texture in Cycles, you would need to recreate your texture in Cycles, or otherwise use a script to convert it to Cycles. See this answer for more information.